This invention relates to a piezoelectric micropump and to methods and apparatuses for pumping fluid in small volumes and at controlled flow rates using a micropump employing a diaphragm and a piezoelectric strip actuator.
Numerous fluidics applications in such areas as medicine, chemistry, and environmental testing exist on a small scale for reasons of sample size, reagent costs, or portability. Cost-effective fluidics including pumps, that are capable and reliable are required for such small scale systems. A number of micropumps are known for delivering small amounts of a fluid to a delivery point. Some of the pumps include piezoelectric actuators. U.S. Pat. No. 4,938,742 to Smits describes a micropump with piezoelectric valves. These valves contain a diaphragm covered by a single layer of piezoelectric material, which limits the control and deflection of the valves. Some of the principles involved in piezoelectric micropumps are described in Piezoelectric Micropump Based Upon Micromachining of Silicone, Sensors and Actuators, 15, 1988 pp. 153-167.
U.S. Pat. No. 4,939,405 to Okuyama et al. discloses a piezoelectric vibrator pump in which a piezoelectric vibrator is mounted in a housing. The vibrator pump does not employ a diaphragm. Instead the vibrator itself is coated with plastic. The pump includes a suction inlet line and a discharge outlet line both of which contain non-return values that alternately open and close in response to the vibration of the vibrator.
U.S. Pat. No. 5,611,676 to Ooumi et al. discloses the use of a cantilevered piezoelectric bimorph. A piezoelectric bimorph has two layers of a piezoelectric material separated by a shim. The application of an electric field across the two layers of the bimorph causes one layer to expand while the other contracts. This causes the bimorph to warp more than the length or thickness deformation of the individual layers.
Another example of a micropump is described in International Patent Application WO 98/51929 to Fraunhofer. Fraunhofer discloses a piezoelectric micropump that is constructed from two silicone wafers each of which includes a valve flap structure and a valve seat structure. The two wafers are juxtaposed and bonded together such that the flap structure in one wafer overlies the valve structure in the other wafer. The micropump is disclosed as being self-priming and suitable for conveying a compressible media.
Commonly assigned U.S. Pat. No. 6,368,079 to Peters describes a micropump which includes a plurality of diaphragm pumping chambers that are actuated by a cantilever mounted piezoelectric strip actuator.
The present invention provides a new and improved piezoelectric micropump.
In accordance to one aspect of the present invention a micropump for pumping a fluid is disclosed that includes a pump body. The pump body includes a fluid inlet channel and a fluid outlet channel, and a pumping chamber. The fluid inlet channel and the fluid outlet channel directly or indirectly communicate with the pumping chamber. The pumping chamber is formed between a plastic diaphragm and a reservoir in the pump body. A piezoelectric strip actuator is attached to the diaphragm such that by applying a voltage to the actuator, the actuator is deformed and the diaphragm is raised or lowered. In accordance with one embodiment of the invention, a reed valve is provided on the inlet and outlet channel. These reed valves open and close the inlet and outlet channels in response to raising and lowering the diaphragm. In one embodiment of the invention, pressures up to about 20 psi and flow rates up to about 100 xcexcl/sec and more typically up to about 50 xcexcl/sec are achieved.
In accordance with the invention, the micropump may include one or more pumping chambers. The term xe2x80x9cpumping chamberxe2x80x9d as used herein includes any chamber formed between an actuated diaphragm and a reservoir in the pump body. The term includes a chamber that functions as a volume accumulator.
In another embodiment of the invention, the micropump includes two or more pumping chambers that may be the same or different volume. In one embodiment, the ratio of the stroke volume of the first pumping chamber to the stroke volume of the second pumping chamber is about 2:1 but the ratio can vary from about 2:1 to 1:1 depending upon the application of the pump.
The diaphragm for the second chamber may be attached to the same piezoelectric actuator that actuates the diaphragm for the first chamber or to a different individually or independently operated actuator. Where the same actuator is attached to both diaphragms, the actuator may be double acting, i.e., the pumping chambers operate 180xc2x0 out of phase with one another. By applying a first voltage to the actuator, the first diaphragm can be raised while the second diaphragm is lowered, and by applying a second voltage (i.e., reversing the polarity of the first voltage), the first diaphragm can be lowered while the second diaphragm is raised.
Micropumps can be designed having sequentially actuated diaphragms and used for a variety of different applications or purposes. In one embodiment, the second pumping chamber may function as a volume accumulator. The outlet from the first pumping chamber directly or indirectly communicates with the inlet to the volume accumulator and the volume accumulator includes a second fluid outlet from which fluid is discharged. Micropumps including two pumping chambers connected in series in this manner can be designed to provide more constant fluid output than a micropump which includes a single pumping chamber. With a micropump having a single pumping chamber, the output occurs in pulses when the diaphragm is lowered or compressed but not when it is raised. If the first chamber is larger than the volume accumulator (e.g., twice as large), a unit of discharge can be achieved with each raising and lowering of the second pumping chamber diaphragm thereby providing more constant output and reducing pulsation.
In another embodiment of the invention, the micropump may be constructed with two or more pumping chambers that are activated sequentially such that fluid is expelled from one chamber as it is drawn into a second chamber. The second chamber volume can vary but for most applications it will be smaller or equal in volume to the first chamber.
In still another embodiment of the invention, the micropump can be constructed with a plurality of pumping chambers having diaphragms that can be actuated individually by dedicated actuators. In accordance with one example of this embodiment of the invention, a micropump can be provided wherein one pumping chamber pumps a liquid composition while the other pumping chamber pumps a gas such as air. The pumped air can be used to purge a line or element in the fluidic flow of the first pumping chamber. In one embodiment, air is used to purge a spray nozzle that is directly or indirectly supplied with liquid from the first pumping chamber.
In accordance with one embodiment of the invention, the reed valve is formed by a film of a flexible polymer that may be either low flex modulus or high flex modulus, such as a KAPTON (aromatic polyimide) film (KAPTON is a trademark of the E. I. DuPont Company). Preferably, the reed valve is formed from a low flex modulus film. In one embodiment a cut out defining a flap which functions as the reed valve is cut in the film. In another embodiment, the film may include a first cut out defining a first flexible flap that functions as an inlet valve and a second cut out defining a second flexible flap that functions as the outlet valve. One of the flaps may be located over a valve seat at the mouth of the inlet channel and the other flap may be located over a valve seat at the mouth of the outlet channel.